SUNSPOTS AND VOLCANIC AND SEISMIC PHENOMENA. 67 



to keep the particles incandescent. The space outside this 

 layer will then be filled with particles at a lower tempera- 

 ture, which act as a screen absorbing and reflecting part 

 of the heat radiated. This cooling goes on and the photo- 

 sphere moves closer to the sun. At length such a stage is 

 reached that the amount of reflected heat overheats this 

 photospheric layer. The vertical temperature gradient 

 rises to such steepness that thermal equilibrium becomes 

 impossible and the overheated vapours break through the 

 photospheric envelope. 



Hence Halm comes to the conclusion that at sunspot 

 maxima there should be minimum radiation into space. In 

 support of this contention he adduces Koppen's temperature 

 curves, 1 which agree with the inverted sunspot curve from 

 1820 to the present time, and the widening of lines in the 

 sunspot spectra at times of sunspot maximum. From the 

 same fact Sir Norman Lockyer deduced the opposite infer- 

 ence, namely that the matter composing the spots must be 

 at a higher temperature at sunspot maxima, and hence the 

 sun must radiate more heat into space at such times. 



Of the above theories, the two last explain most facts, 

 but Herr Halm weakens his case by drawing inferences 

 from the meteorological curve of Koppen. The amount of 

 cloud, moisture of the atmosphere, and the prevalence of 

 cyclones probably give a better clue to the amount of solar 

 heat received by the earth than mere temperature curves. 

 The temperature at the earth's surface is modified by so 

 many local causes, precipitation, evaporation, humidity, 

 and barometric pressure, that it can scarcely give us any 

 idea of the quantity of heat received from the sun. 



The relations between sunspot intensity and terrestrial 

 phenomena which have already been shown to exist, (by Dr. 



1 Reproduced in Herr Halm's paper, see Nature, No. 1685, Vol. lxv., 

 p. 353. 



